Hi, I'm new to marbling, and on the videos I've watched, people run some newspaper over the size to clean it after each use. I have done my best to do the same, but am finding it gets quite murky quite quickly. Can anyone give me any suggestions about how to keep it clean? I'd like to be able to use it for longer than I'm currently managing.

Do you use a wooden skimmer board prior to the newspaper skim? It sounds as though your pigments are not floating properly and quickly dropping beneath the surface. Thus, polluting the size. I suggest taking a class with a professional marbler to learn correct procedures or find a good book which explains the technique accurately. Online videos sometimes omit important steps.